Preservatives, especially CCA (chromated copper arsenate), CCB (chromated copper borate), ACA (ammoniacal or amine copper arsenate), and the like, have found extensive usage in wood, fox the purpose of extending the useful life of the wood by their incorporation into same. Due to the highly toxic effects of these preservatives, even at very low levels, their use in the treatment of large quantities of wood, poses environmental concerns for the manufacturers. Numerous modifications of the general process of impregnating the wood with these preservatives have been developed in order to produce an economically feasible product while still meeting the strict environmental standards imposed upon the industry as a whole.
The principal problem in the overall process is the need to fully fix the preservative within the wood during the treatment process in order to minimize or eliminate the subsequent contamination of the area around the wood caused by the runoff of excess, or non-fixed, preservative. Unless preservative is fixed within the wood, rain water will wash or leach preservative components out of the treated lumber and onto or into the soil, where they may be carried into groundwater or waterways, thus contaminating the surroundings as these metals accumulate. Present EPA regulations define any soil or water that tests above 5.0 ppm for either arsenic or chromium as "hazardous waste", and designates the site where they are located as "contaminated". When such conditions arise, the treaters and/or their customers are required to decontaminate the site, and dispose of the "hazardous waste" at approved landfills.
The so-called "Drinkard Process" developed by William F. Drinkard, Jr. and disclosed in U.S. Pat. No. 4,927,672, used a hot-water fixation step to accelerating fixing of fixable wood preservatives in freshly impregnated wood. The process involves contacting preservative-impregnated wood with an aqueous heated liquid medium preheated to at least 100.degree. F., raising the temperature of the wood from ambient to from 100.degree. F. to 240.degree. F., and maintaining both liquid contact and raised temperature of the wood for a period of time from 20 minutes up to 2 hours, whereby complete fixation occurs in less than 48 hours. It has recently been discovered that this process, while an improvement over other existing processes using steam or heated air to fix the preservatives, results in a high amount of sludge formation in the fixation medium and/or a product that is not acceptable to the consumer due to its disfigurement by the sludge chemicals.
Another process available to the wood preservative industry, but not generally used because of its practical limitations, is disclosed in U.S. Pat. No. 4,303,705, which utilizes heating under pressure with the treating solution in order to effect fixation of the preservatives. This process, however, requires the wood to be retained within the treating vessel for the additional amount of time needed for fixation, resulting in a prolonged cycle time, and there is difficulty in maintaining pressure during removal of the treating solution and then filling with the heating medium. The objective of this process is to produce a treated wood of lower weight than the prior art methods, rather than to improve fixation. It is probable that sludging difficulties similar to those discovered with the so-called "Drinkard process" would also occur with the process disclosed in U.S. Pat. No. 4,303,705.